BROWN dwarfs lie in the mass range between the most massive Jupiter-like planets and the least massive stars, They are much less luminous than stars, and so may provide a fraction of the baryonic dark matter in our Galaxy, Only one unambiguous detection of a brown dwarf has been made to date(1-6)-Gl229B, a low-mass companion to the nearby star Gl229A, The detection(4) of strong methane-band absorption in the spectrum of Gl229B, a feature restricted to cool substellar objects(5-9), lends weight to the idea(7) that differential methane-band imaging (the subtraction of an image taken in the methane band from a continuum-light image taken in the same spectral region) should provide an efficient method for detecting brown dwarfs. Here we demonstrate the potential of this approach by obtaining an image of Gl229B with less than two minutes of integration time. This technique promises efficient detection of both isolated brown dwarfs in crowded regions, and brown dwarfs orbiting close to their primary stars.